Spinal rats and rats with an intact neuraxis received an intrathecal injec- tion of an enantiomer of baclofen. The R-enantiomer was 100--1000 times more potent ...
J. Neural Transmission 60, 63-67 (1984)
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Nmwal Tmmm~h~/on 9 by Springer-Verlag 1984
Stereoselectivity of Spinal Neurotransmission: Effects of Baclofen Enantiomers on Tail-flick Reflex in Rats D.F.Smith The Psychopharmacology Research Unit, Aarhus University Institute of Psychiatry, Psychiatric Hospital, Risskov, Denmark With 2 Figures Received February 23, 1984; revised May 21, 1984
Summary Spinal rats and rats with an intact neuraxis received an intrathecal injection of an enantiomer of baclofen. The R-enantiomer was 100--1000 times more potent than its antipode in inhibiting the tail-flick reflex, both in intact rats and in spinal rats. Spinalization enhanced the inhibitory effects of both enantiomers without altering their dose-response relations. The findings show that baclofen enantiomers have direct actions on stereoselective spinal mechanisms and that spinalization fails to alter the stereoselectivity of spinal mechanisms towards the enantiomers.
Introduction Stereoselectivity plays a basic role in many aspects ofneurotransmission and in the effects o f many treatments in the nervous system (Smith, 1984). The stereoselectivity of neuronal processes can be studied readily using enantiomers (Lehmann F., Rodriques de Maranda, and Arigns, 1976). Transection of the spinal cord leads to potentiation of spinal reflexes (Zemlan, Corrigan, and Pfaff,, 1980), and it is of interest to know whether that potentiation reflects changes in the stereoselectivity of spinal neurotransmission. The enantiomers of baclofen were used in the present study to determine whether spinalization influences the stereoselectivity o f mechanisms governing the tailflick response.
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D.F. Smith
Materials and Methods Intrathecal (i.t.) catheters (PE-10) were implanted into the lumbar subarachnoid space in 192 male Wistar rats as previously described (]ensen and Smith, 1982). Three days later, a laminectomy was performed at T6_8 in 88 of the rats anesthetized with halothane and a 2 mm portion of the spinal cord was removed gently by suction. Tail-flick latency was measured on the next day as previously described (]ensen and Smith, 1982). The intensity of the thermal stimulus was adjusted in preliminary experiments to a level that produced a tail-flick response after 3-4 sec in normal rats. Baseline tail-flick latency was recorded first. Then, each rat received an i.t. injection of an isotonic saline solution (15/~1) containing an enantiomer of baclofen (R-I-l- or S-[+]-fl-[4-chlorophenyl]-y-aminobutyric acid HC1) (Fig. 1). Tests took place again 5, 10, 30, and 60 rain later for each rat. There were eight rats in each group and each rat received only one dose of one enantiomer. An all-or-none method was used to determine whether the tailflick reflex was inhibited by the enantiomers; failure to move the tail away from the thermal stimulus within 10 sec after its onset indicated that the reflex was inhibited. The doses used covered the response range from 0% inhibition (i.e. all of the 8 rats showed the tail-flick reflex within 10 sec) to 100% inhibition (i.e. none of the 8 rats showed the tail-flick reflex within 10 sec). Student's t-test was used to determine whether baseline latencies differed significantly between intact and spinal rats. Stepwise logistic regression (Dixon, 1983) was used to analyze the effects of baclofen enantiomers on the tail-flick reflex in intact and spinal rats. That analysis was based on logic transformations. The statistical model used was such that a significant effect for the drug x spinal interaction would indicate that transection of the spinal cord altered the stereoselectivity of the mechanisms governing the effects of baclofen enantiomers on the tail-flick reflex.
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Fig. 1. Structural diagrams of the enantiomers of baclofen (R-isomer at left, Sisomer at right)
Results Baseline tail-flick latency was significantly shorter in spinal rats than in intact rats (mean + S.D. = 2.6 + 0.3 sec and 3.8 + 0.5 sec, respectively) (p < 0.001). Fig. 2 shows the dose-response curves for the effects o f baclofen enantiomers o n the tail-flick response. T h e inhibitory effects o f the enantiomers differed significantly; the Re n a n t i o m e r was more p o t e n t than the S-enantiomer at all times
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Fig. 2. Effect of baclofen enantiomers on the tail-flick response in spinal rats (unfilled symbols) and in rats with intact neuraxis (filled symbols). An enantiomer ofbaclofen (R-isomer=circles, S-isomer=squares) was injected intrathecally and the tail-flick response latency was measured 5,10, 30 and 60 min later. The dose-response curves show the percentage of rats (8 per group) that failed to move their tail away from the thermal stimulus within 10 sec (cutoff time). Regression lines are for spinal rats (dashed lines) and for rats with intact neuraxis (solid lines)
(p < 0.001). Inspection of the data shows the potency difference between the enantiomers to be about 100-1000 fold. The tail-flick reflex was inhibited by significantly lower doses of the enantiomers in spinal rats than in rats with intact neuraxis at all times (p's
